1. Field of the Invention
The present invention relates to a current source cell which output predetermined current in accordance with a digital input signal and to a D/A converter using the same current source cell, and more particularly, to a sufficiently precise current source cell and a D/A converter using the same current source cell.
2. Related Background Art
A D/A converter (digital/analogue converter) is used for inputting calculated and processed digital data to an apparatus which is operated by an analogue signal.
In a conventional D/A converter to which a current source cell having an analogue switch connected to a current mirror circuit is connected in parallel, parasitic capacity exists in a transistor constituting the D/A converter. Thus, there is a problem that switching noise is generated in output waveform when the analogue switch is switched.
Further, with the requirement of low voltage and high precision of recent years, since the conventional current source cell does not have sufficient output resistance, there is a problem that the linearity is degraded and output waveform is distorted.
There is a known D/A converter in which a normally conducting transistor is serially connected to an analogue switch, thereby suppressing the generation of switching noise and to prevent output waveform from being distorted by switching noise. For example, see Japanese Patent Publication No.2573427, especially page 5 and FIG. 6 thereof.
The D/A converter disclosed in Japanese Patent Publication No.2573427 includes a decoder for decoding a digital input signal to output a control signal, a plurality of current source cells for outputting predetermined current in accordance with a control signal which is output from the decoder, and a resistor for converting a sum of output current of the current source cells into voltage.
The current source cell includes first and second transistors which are complementarily switched by a control signal output from the decoder, a constant current source which is commonly connected to one ends of current paths of the first and second transistors, and third and fourth transistors which are respectively connected to the other ends of the current paths of the first and second transistors and first and second output terminals, and which are in a normally conducting state.
Reference voltage which is sufficiently greater than threshold voltage is supplied to gates of the third and fourth transistors, and when voltage Vds between drain and source of the third and fourth transistors is sufficiently high, the third and fourth transistors are conducting in a saturation region.
When the third and fourth transistors are n-type MOS transistors, power source voltage Vdd is usually supplied to the gate, and when the third and fourth transistors are p-type MOS transistors, earth potential Vss is supplied to the gate.
The normally conducting third and fourth transistors are serially connected to the switching first and second transistors. Therefore, damping effect is generated by conduction resistance of the third and fourth transistors with respect to parasitic capacity of the transistors, and switching noise caused by switching of the first and second transistors is suppressed.
Further, the third and fourth transistors are serially connected to the constant current source through the first and second transistors. Thus, if the third and fourth transistors are conducting in the saturation region, the output resistance of the current source cell is greater than that of the constant current source only, and even if the output voltage of the D/A converter is varied, the D/A converter has excellent linearity.
In the case of the D/A converter disclosed in Japanese Patent Publication No.2573427, however, if the power source voltage is reduced by reduction of semiconductor device in size and the operation amplitude of output voltage of the D/A converter is relatively increased, the voltage Vds between drain and source of the third and fourth transistors becomes insufficient, the third and fourth transistors conduct in the linear region not in the saturation region and thus, there is a problem that the output resistance of the current source cell is largely reduced, the linearity of the D/A converter is degraded and the output waveform is distorted.
In order to suppress the generation of the switching noise caused by switching of the first and second transistors, to secure excellent linearity and to prevent output waveform from being distorted also in a region where the output voltage amplitude of the D/A converter is increased, it is necessary that the third and fourth transistors always conduct in the saturation region and the current source cell has extremely high output resistance.
When the first, second, third and fourth transistors are p-type MOS transistors and earth potential Vss is supplied to the gates of the third and fourth transistors, however, in order to operate the third and fourth transistors in the saturation region, it is necessary to set the output voltage amplitude with respect to earth potential Vss smaller than the absolute value of the threshold voltage of the third and fourth transistors based on a following inequality representing the operation of transistor in the saturation region:Vds>Vgs−Vthwhere Vds represents Voltage between drain and source, Vgs represents Voltage between gate and source, and Vth represents threshold voltage.
At present, the threshold voltage of usually used transistor is about 0.5V, and is about 0.7V even if substrate bias effect generated from a difference between source potential and bulk potential of the third and fourth transistors is taken into consideration.
This is not suitable for a D/A converter in which large amplitude operation of output amplitude of about 1.0V is required.
If the gate length is increased to increase the output resistance of the third and fourth transistors, since the parasitic capacity in the output terminal of the D/A converter is increased, there is a problem that frequency characteristics of the D/A converter are degraded.
Thus, a sufficiently precise current source cell also at the time of large amplitude operation and a D/A converter using such a current source cell are desired.